• The 24-hour urinary 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) output was used as the basis for selection of tricyclic antidepressant drug therapy for the depressed patient population treated by one psychiatrist over a period of ten months in a psychiatric clinic. Use of MHPG output level as the criterion for drug selection resulted in significantly better clinical results than had been obtained previously by the same psychiatrist using more traditional selection methods on a similar depressed patient population. A correlation was noted between patients' pretreatment MHPG output levels and three symptoms of depression (guilt, agitation, and diurnal variation) as measured on the Hamilton Rating Scale for Depression.